Pre- and post-landing response of the parasitoid Encarsia formosa to whitefly hosts on Gerbera jamesonii

One of the factors that may complicate biological control of the greenhouse whitefly on Gerbera jamesonii by Encarsia formosa is the rosette shape of this ornamental, which differs from the vertical shape of most vegetable pl ants (cucumber, egg plant, tomato, etc.). Therefore, host-habitat location and the behaviour prior to landing on uninfested and infested leaves was st udied. Attraction of E. formosa from a short distance by infested leaves co uld not be detected: the parasitoid females landed at random on uninfested and infested leaves. After the first landing, a redistribution of the wasps occurred on the leaves. After 24 h three times as many wasps were found on the infested leaves than on uninfested ones. In a dispersal experiment wit h four plants, E. formosa appeared to have no preference for landing on lea ves of the medium age class, which is the age class on which most of the wh iteflies in a suitable stage for parasitism occur. Twenty percent of the pa rasitoids were found on the plants 20 min after releasing them. These resul ts were independent of the plant cultivar and the host density on the plant s. In the course of 8 h, the number of E. formosa females recovered from pl ants increased linearly, and this increase was greater on plants where host s were present and also greater on the plant cultivar with the lowest trich ome density. After 24 h, the percentage of females was highest (56%) on pla nts with the highest host density. E. formosa females were arrested on leav es where hosts were present. Contrary to our expectation, the results from the two G. jamesonii cultivars that differed strongly in leaf hairiness wer e not significantly different in most experiments. Only at the high host de nsity was parasitism found to be lower on the cultivar with the higher hair density. Parasitoids may walk on top of the 'hair coverlet' of cultivars w ith high trichome density and, therefore, be hampered less than expected.